1. Field of the Invention
The present invention pertains to the field of integrated circuit technology and, more particularly, to high performance thin-film materials for use in ferroelectric switching capacitors. Still more specifically, a single liquid precursor solution is used to manufacture a thin-film ferroelectric material having multiple layers that present an oxygen octahedra superlattice.
2. Description of the Prior Art
Ferroelectric layered perovskite-like materials are known, and have been reported as phenomenonological curiosities. The term "perovskite-like" usually refers to a number of interconnected oxygen octahedra. A primary cell is typically formed of an oxygen octahedral positioned within a cube that is defined by large A-site metals where the oxygen atoms occupy the planar face centers of the cube and a small B-site element occupies the center of the cube. In some instances, the oxygen octahedral structure may be preserved in the absence of A-site elements. The reported annealing temperatures for these materials often exceed 1100.degree. C. or even 1300.degree. C., which would preclude their use in many integrated circuits.
Researchers have made and tested a large number of materials, and those materials found to be ferroelectric or to have high dielectric constants have been included in lengthy lists. See for example, Appendix F of Principles and Applications of Ferroelecirics and Related Materials, by M. E. Lines and A. M. Glass, Clarendon PrEss, Oxford, 1977, pp. 620-632. Ferroelectric materials are known to switch polarization states in the presence of an applied electric field, and can retain the switched polarization state for indefinite periods of time after the field is removed. Additional background information can be obtained from copending application Ser. No. 07/965,190 filed Oct. 23, 1992, which is hereby incorporated by reference herein to the same extent as though fully disclosed herein.
Prior ferroelectrics are afflicted with high fatigue rates that preclude their widespread commercial acceptance. That is, under repeated switching, the polarizability decreases to less than 50% of the original polarizability over a useful life of an unacceptably short duration. Moreover, when a series of pulses of the same sign are applied to the prior art materials, they take a set, or an imprint. See for example the article "Anomalous Remanent Polarization In Ferroelectric Capacitors" by Norman E. Abt, Reza Moazzami, and Yoav Nissan-Cohen, in Integrated Ferroelectrics, 1992, Vol 2, pp. 121-131, which discusses the .DELTA.PO phenomenon.
Two layered perovskite-like ferroelectric materials, namely, bismuth titanate (Bi.sub.4 Ti.sub.3 O.sub.12) and barium magnesium fluoride (BaMgF.sub.4), were unsuccessfully tried in a switching memory application as a gate on a transistor. See "A New Ferroelectric Memory Device, Metal-Ferroelectric-Semiconductor Transistor", by Shu-Yau Wu, IEEE Transactions On Electron Devices, August 1974, pp. 499-504, which relates to the Bi.sub.4 Ti.sub.3 O.sub.12 device, and the article "Integrated Ferroelectrics" by J. F. Scott, C. A. Paz De Araujo, and L. D. McMillan in Condensed Matter News, Vol. 1, No. 3, 1992, pp. 16-20.
Neither of the above two devices was suitable for use in a memory. In the case of the Bi.sub.4 Ti.sub.3 O.sub.12 device, the ON state decayed logarithmically after only two hours. Similarly, both states of the BaMgF.sub.4 device decayed exponentially after a few minutes, but the rapid dissipation of the "polarized" state creates significant questions whether the material actually functioned as an electret, not a ferroelectric. See "Memory Retention And Switching Behavior Of Metal-Ferroelectric-Seniiconductor Transistors", by S. Y. Wu, Ferroelectrics, 1976 Vol. 11, pp. 319-383. Thus, the prior art suggests that materials degradation is an insuperable obstacle to commercial acceptance.
Yet another problem with ferroeletric materials is that the ferroelectric phenomenon is temperature sensitive. The polarization magnitude varies greatly across a temperature range, and may not even occur at all depending upon the temperature.